A Selwyn is pretty much when someone tells you they're going to do something, then either doesn't do it, or just completely fails in that sense, usually ruining that section of the day or just making it annoying to be around.
It is a worse version of admitting defeat or a gg, which is what the concept was first derived from.

Guy #1 "Remember that time Selwyn did a Selwyn on us?"
Guy #2 "Don't remind me."